The heat of adsorption of benzene on clean Pt(111) at 300 K is measured calorimetrically and found to decrease with coverage (θ) as (197 -48θ -83θ 2 ) kJ/mol. Saturation coverage (θ ) 1.0) is 2.3 × 10 14 molecules/cm 2 . Sticking probabilities of benzene on Pt(111) were measured by mass spectrometry, giving an initial value of 0.97 and showing Kisliuk-type behavior with increasing coverage that implies there is a precursor to sticking with a ratio of its hopping rate to its desorption rate of ∼28. Benzene adsorbs transiently on the benzene-saturated surface at 300 K with a trapping probability of 0.90 and a heat of adsorption of 63-73 kJ/mol.

A calorimeter for measuring heats of adsorption of large molecules on single crystal surfaces is described. It extends previous instrumentation for single crystal adsorption calorimetry by adding the capability for measuring larger (lower vapor pressure) molecules. This is achieved using a chopped and collimated (∼4 mm diameter) molecular beam capable of stable 100 ms pulses of low vapor pressure substances, and a line-of-sight modification of the King and Wells method for measuring their sticking probabilities at the single crystal’s surface. The heat input to the single crystal due to adsorption is detected using a pyroelectric polymer ribbon pressed against the back of the single crystal, following our previous calorimeter design. Measurements of benzene adsorption on Pt(111) prove the capability to produce a highly stable beam of flux ∼2×1014 molecules/(cm2 s) and measure adsorption energies with an absolute accuracy of ∼5% and a pulse-to-pulse standard deviation of 2 kJ/mol.

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